There are known techniques based on the masking theory that reduce the unpleasantness of a noise felt by a user. For example, PTL 1 discloses a noise eliminating device that makes a noise, such as a sound of gears meshing with each other, less perceivable to the ears of a user by outputting a white noise having slightly smaller volume than the noise.
PTL 2 discloses an active vibration noise suppressing device that actively suppresses a noise produced by a vehicle by outputting a control sound that is in opposite phase to the noise.